High-frequency behavior of magnetically soft wires

In addition to opening the possibility of interesting applications, the study of the behavior of soft wires up to GHz frequencies permits a comparison between theory and experiment, which is more difficult at low frequencies, at which domain wall motion is possible. Obviously, the importance of this comparison depends upon the sophistication both of the experiments and of the models. Agreement between experimental measurements of the form of impedance as a function of frequency and of field and theoretical models is excellent, especially at high frequencies. For samples with low enough magnetization to permit the observation of both ferromagnetic resonance and ferromagnetic antiresonance in impedance, it is possible to deduce a number of fundamental properties of the material. However, the observed amplitudes of the maxima and minima of impedance or of permeability are in disagreement with the predictions of the simplest models. In order to understand the cause, or causes, of this disagreement, it is desirable to improve both the experimental and theoretical situations. We discuss the present state of experiment and of theory, and consider the comparison between theoretical predictions and experimental results. While there are some successes in modelling, there remain discrepancies. To explain these, the possible effects of surface anisotropy, damping mechanism, and anisotropy dispersion on the absolute value of impedance, as a function of field and frequency are investigated. The role of the last of these is particularly subtle.